1. Field of the Invention
The invention relates to a device for holding a gripper for an electrical component on a fitting head, where the gripper can be fitted detachably to an end section of a substantially cylindrical shaft of the fitting head.
2. Description of the Related Art
It was previously common to plug grippers designed as suction pipettes and having a hollow cylindrical centering section onto a cylindrical end, section of a rotatable shaft of the fitting head. In such configurations, a flat annular locating groove is let into the inner side of the socket section. An elastomeric O-ring is inserted into an outwardly open annular groove of the shaft and engages under tension in the locating groove. The annular groove and the locating groove are offset axially to such an extent that the gripper is pressed in the axial direction against a stop on the shaft, as a result of which the gripper axis and the shaft axis are aligned parallel to each other. A short centering section of the gripper bears with a close fit on a corresponding cylindrical section of the shaft; this centers the gripper in the radial direction on the shaft.
A pipette axis angular deviation from the shaft axis impairs the circular running of the pipette tip holding a component. Before being placed on a printed circuit board, the component has to be rotated from a fetching position into a placement position, making possible a corresponding lateral offset. For very small components, eccentricity of the pipette tip can lead to these elements being sucked up eccentrically and drawn up into the suction channel, thus disrupting the pneumatic functioning of the fitting head.
The O-ring has relatively large production tolerances, which lead to the plug-on and pull-off forces varying over a wide range. Since the grippers have to be changed relatively frequently, depending on the components to be gripped, the O-ring has to be regreased frequently and carefully (with considerable expenditure on maintenance) in order to avoid correspondingly high frictional wear. Even the carefully maintained O-ring is subjected to residual wear, which necessitates periodic replacement. Since the frequency of change of the individual pipettes cannot be monitored automatically, it is also correspondingly difficult to coordinate the maintenance intervals in such a way that the pressing function is fulfilled reliably at any time, so that an angular deviation between the axes of the shaft and of the suction pipette cannot always be avoided.
Japanese patent document JP 2067799 A discloses a holding device for fixing a suction pipette to a fitting head, which has a hollow cylindrical centering socket, permitting insertion of a convex cylindrical connecting pin of the suction pipette. The connecting pin has an annular groove, in which radially deflectable locating spheres belonging to the fitting head engage and, in the event of a pipette change, are released by pushing a clamping bush back. An active locating mechanism of this type requires a relatively high mechanical outlay, with a correspondingly large external diameter in the area of the holding section. The requisite slimness of the pipette can be achieved only by lengthening it, but this results in a further removal of the pipette tip from the holding section, which reduces its positional accuracy.
The invention is based on the object of permitting secure and accurate fixing of the gripper on the shaft with little outlay on production and maintenance.
This object is achieved by a device for holding a gripper for an electrical component on a fitting head, comprising:
a substantially cylindrical shaft of the fitting head, comprising a locator configured to project radially from the shaft and configured as peripherally distributed fixed short locating caps relatively permanently connected to the shaft, the shaft configured to accept the gripper fixed detachably to an end section of the shaft in an axial direction; and a stop against which the gripper is pressed in the axial direction;
the gripper further comprising a hollow cylindrical centering section in an area of the shaft and resting on the shaft; and an annular locating groove located close to the centering section, the groove configured to engage the shaft locator, and the groove being offset axially from the locator when the gripper is pressed in the axial direction against the stop of the shaft.
The locating caps can be arranged in a precisely defined position and dimension on the shaft. They can have an abrasion-resistant and smooth surface, which is subject to low wear. The depth of the locating groove is so low that the required mutual deflection lies in the range of elastic compliance. In particular, the axial offset between the locating groove and the locating caps can be defined to a precise extent, so that the axial pressing forces can be limited considerably more accurately. The plug-on and pull-off forces can likewise be limited to a closer tolerance band, resulting in a reduction in the average actuation force.
The locating caps may be designed as spherical domes. These locating can be configured to result in point-like contact between the shaft and the gripper, which benefits the mutual deflection during location.
The invention may also comprise a shaft annular groove of the shaft and metallic locating spheres configured to be inserted into and affixed to the shaft annular groove, the locating spheres having locating caps formed on them. This annular groove may be formed exactly concentrically with the cylindrical centering section, so that the metal spheres can be arranged precisely in their radially installed position.
These locating spheres may be designed as steel spheres for ball bearings and may be configured to be fixed in the annular groove. This can be obtained cost-effectively as standard parts of high accuracy, high strength, and good sliding characteristics are available. As a result of lining the entire annular groove, the spheres may be firmly connected to the shaft, possibly using an adhesive bonding. This material may be of such a viscous consistency that it adheres in the annular groove without any further aid before being cured. The locating spheres can be inserted into this material by simply being pressed in. A sealing material (e.g., silicon rubber) may also be provided to line the annular groove.
The inventive device may further comprise a socket-like annular wall of the gripper in a fixing area configured to engage around the shaft from outside, a wall thickness of the annular wall being so low that it deforms under stress of the locating caps to form a round corner. The gripper wall can be bent out of its cylindrical shape to such an extent that sufficient compliance can be achieved without permanent deformation during the locating operation. It is particularly beneficial in this case if between three and five spheres are distributed in the annular groove. This results in secure centering with adequate compliance.